This is down to the use of Mail::CheckUser in flo/plugin/Account/Subscribe.pm — this is what the CBL says:

The Perl CheckUser module defaults to improper “HELO” and “MAIL FROM” strings: “localhost.localdomain” and “check at user.com” respectively. The former is illegal, the latter impersonates user.com – they probably don't like that. [Besides, by not using your own domain, some spam filters will lie to your RCPT TO.]

You will need to change $Helo_Domain = to be “<DNS name of your server>” and change $Sender_Addr to be something in your domain (eg: “[email protected]<mydomain>”)